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https://issues.apache.org/jira/browse/HIVE-5093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13744063#comment-13744063
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Gopal V commented on HIVE-5093:
-------------------------------

[~appodictic]: HIVE-3562 is actually doing this in the map-side aggregation 
with a java.util.SortedMap. 

I wrote the combiner version for memory efficiency, which is relatively simple 
as well.
                
> Use a combiner for LIMIT with GROUP BY and ORDER BY operators
> -------------------------------------------------------------
>
>                 Key: HIVE-5093
>                 URL: https://issues.apache.org/jira/browse/HIVE-5093
>             Project: Hive
>          Issue Type: Bug
>          Components: Query Processor
>    Affects Versions: 0.12.0
>            Reporter: Gopal V
>            Assignee: Gopal V
>         Attachments: HIVE-5093-WIP-01.patch
>
>
> Operator trees of the following structure can have a memory friendly combiner 
> put in place after the sort-phase 
> "GBY-LIM" and "OBY-LIM"
> This will cut down on I/O when spilling to disk and particularly during the 
> merge phase of the reducer.
> There are two possible combiners - LimitNKeysCombiner and 
> LimitNValuesCombiner.
> The first one would be ideal for the GROUP-BY case, while the latter would 
> more useful for the ORDER-BY case.
> The combiners are still relevant even if there are 1:1 forward operators on 
> the reducer side and for small data items, the MR base layer does not run the 
> combiners at all.

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